1. Field of the Invention
The present invention relates generally to antenna systems, particularly for use in measurement of position or location by means of electromagnetic signaling.
2. Background of the Invention
Magnetic antennas, particularly loopstick antennas, are often used for reception of low frequency signals. FIG. 1 is a circuit diagram showing a prior art loopstick antenna 104. Prior art loopstick antenna 104 comprises primary coil 131, ferrite rod 135, secondary coil 133 and tuning means 132. In alternate embodiments, ferrite rod 135 may be iron, some other ferromagnetic material, or even a non-ferromagnetic material to support primary coil 131. Ferrite rod 135 may also be a bar, cylinder, prism, or other geometric form suitable for supporting primary coil 131. In alternate embodiments ferrite rod may be dispensed with altogether if primary coil 131 is sufficiently stiff to maintain a suitable mechanical shape. Secondary coil 133 provides coupling into primary coil 131 and means by which prior art loopstick antenna 104 may further couple to an RF device.
FIG. 2 is a schematic diagram illustrating a prior art loopstick antenna 104. Primary coil 131 generally comprises turns with normals co-located with loopstick axis 205. Loopstick axis 205 is not only an axis of ferrite rod 135, but also a null axis for prior art loopstick antenna 104. Pattern of prior art loopstick antenna 104 lies generally along pattern axis 207. Pattern axis 207 is substantially orthogonal to loopstick axis 205.
Null axis is a name for an axis referring to a far field pattern for the antenna and is referred to as null axis even though near field coupling may be achieved along the null axis. The null axis is typically the coil winding axis of a conventional cylindrical coil as well as the direction of the antenna moment vector.
A variety of prior art seeks to determine position using signal amplitude, or equivalently signal strength. These efforts are confounded by the impact of typical multi-path heavy environments which act to constructively and destructively combine signals in such a fashion as to render typical signal strength ranging schemes wildly inaccurate. These impacts differ for different building types. Different building types pose different propagation environments for electromagnetic signals and effect a signal strength ranging system in different ways.
Furthermore, existing amplitude or signal strength positioning schemes tend to use sub-optimal antenna arrangements that seriously impact their performance. Existing antenna arrangements are large and bulky or small and inefficient. Other existing antenna arrangements are prone to undesirable coupling to a mobile asset or person being tracked.